The present invention relates to a light-sensitive mixture that contains a diazonium salt polycondensation product, a particular type of crosslinking compound and a polymeric binder, which mixture is especially suitable for the preparation of printing plates.
German Offenlegungsschrift No. 2,024,244 (corresponding to U.S. Pat. No. 3,867,147) has disclosed the use, for the preparation of printing plates, of light-sensitive recording materials comprising mixed condensation products of diazonium salts capable of condensation and other non-light-sensitive compounds capable of condensation, if appropriate in combination with polymeric binders. These light-sensitive materials are distinguished by a high light-sensitivity and long print runs. The run is particularly long when waterinsoluble binders, such as polyvinyl formal, are used. But such layers can be cleanly processed only with those developers that contain a considerable proportion of volatile organic solvents.
German Offenlegungsschrift No. 3,130,987 has disclosed similar mixtures that contain styrene/maleic anhydride copolymers as the binder and that can be developed with purely aqueous alkaline solutions. As a rule, these binders have acid numbers far above 100. The printing plates obtained with these mixtures give shorter print runs, however, than those that contain binders insoluble in .aqueous alkali.
German Offenlegungsschrift No. 3,036,077 has disclosed corresponding mixtures that contain polymers with alkenylsulfonylurethane side groups as the binder. These mixtures can also be developed with purely aqueous solutions. When processed to give planographic printing plates, however, they give light-hardened layers characterized by an ink absorption that is not satisfactory and a length of print run that is again limited.
European Patent Application No. 152,819 describes similar mixtures that contain a diazonium salt polycondensation product and a polymeric reaction product obtained from a polymer containing hydroxyl groups and intramolecular anhydrides of polycarboxylic acids, preferably dicarboxylic acids. The recording materials prepared from these mixtures can also be developed with aqueous alkaline solutions and are distinguished by advantageous copying and printing properties, in particular by a long print run as compared with other planographic printing plates that can be developed under aqueous-alkaline conditions.
Since the aim in principle is always a further increase in the length of print runs, attempts have been made to increase the print run performance of planographic printing plates by additional processing measures. For example, German Offenlegungsschrift No. 1,447,992 describes a process for the preparation of offset printing forms of increased print run performance, wherein the exposed and developed printing form is reheated for a certain period, such as for 20 minutes to 240.degree. C. This effect is restricted to those layers containing novolaks as the binder. A significant improvement is obtained, moreover, only with positive-working layers based on naphthoquinone-diazides.
German Offenlegungsschrift No. 3,246,037 describes light-sensitive mixtures that contain various positive- or negative-working light-sensitive compounds, alkali-soluble binders and a crosslinking agent which can bbe activated thermally. The crosslinking agent used is, for example, hexamethylolmelamine and its methyl ethers Diazonium salt polycondensation products are also mentioned as light-sensitive, negative-working compounds. Here again, the best effect is obtained with positive-working mixtures, and the storage life of the mixtures is limited.